Coupled circuits for high impedance aerials



@QQ EL? I@ C. W. r- TGGLETON 45s-5697272 COUPIQED CIRCUITS FOR HIGH IMPEDANCE AERIALS Filed Jan. 5, 1946 ATTORNEY Patented Dec. '21,r 1948 COUPLED CIRCUITS FOR HIGH IMPEDANCE.

AERIALS Charles William Eggleton,

Waltonori-Thames,

England, assigner to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application January 5, 1946, Serial No. 639,275y

In Great Britai n ASeptember 6, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires August 27, 1965 4 Claims.

This invention relates to coupling circuits and has particular, but not exclusive, reference to coupling circuits employed in radio receivers in which it is required to couple a high impedance aerial, such as short vertical rod, tightly to the tuned input -circuit of the receiver so that the overall transfer of energy is as high as possible to compensate for the small dimensions of the aerial. Receivers employing aerials of this form are frequently employed in motor vehicles.

A known form of circuit for this purpose oomprises a tunable parallel resonant circuit having an inductance and variable capacity and in which the stray capacity associated with the aerial which would otherwise restrict the tuning range is counteracted by a series capacity. In such an arrangement in order to compensate for the loss of coupling due to the eiect of the series capacity a shunt inductance is provided which forms,lwith the stray capacity, a parallel resonantcircuit which is tuned to about 550 kilocycles per second so as to improve the performance of the aerial at the low frequency end of the medium waveband. A coupling circuit of this form is dis-closed in British Patent No. 383,987.

The object of the present invention is to provide a coupling circuit of this general type which can be employed for tuning in more than one waveband.

According to the invention a coupling circuit is provided comprising a resonant tunable circuit and a parallel resonant circuit, the capacity of which is constituted in whole or in part by stray capacity, coupled to said tunable resonant circuit by a capacity and wherein switching means are provided for switching the inductance of said parallel resonant circuit into circuit with the tuning capacity of said tunable circuit so that the inductance of said parallel resonant circuit can be employed as the inductance of the tunable circuit on another waveband.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawing which illustrates a circuit in accordance with one form oi the in vention, Figs. 1 and 2 showing the circuit connections with the switch in the medium and long wave positions, respectively.

The invention is shown in the drawing as applied to a coupling circuit for coupling a high impedance aerial to the tuned input circuit of a radio receiver of the car type. The reference numeral i indicates the aerial and the stray shunt capacity associated with the aerial is indicated by the l y' dotted capacity t between aerial I' and ground 8.

Connected in shunt across the stray capacity 4 is an inductance E5, and a second inductance 2 is connected at one 'end to ground through a lead 9 and at the other end to the high potential side of coil 5 through a series coupling capacity 5. A variable capacity 3 for tuning the input circuit of the receiver is connected between ground lead 53 and a lead I il and is adapted for selective connection across either coil t or coil 2 by way of a double-pole switch '1. One pole I I of the switch is connected to the high side of capacity 3 through lead lil and is adapted to make selective contact with a fixed contact I2 which is connected through a lead i 3 to the high potential end of coil 2 or with a iixed contact I4 which is connected through a lead I5 to the high side of coil 6. The other pole It of the switch is connected by a lead wire il to the ground lead 9 and is adapted to make selective contact with a blank contact I8 or with the xed contact I2.

In the medium waveband position of the switch as shown in Fig. l the aerial is tuned by inductance 2 and the shunt variable capacity 3. In order to reduce the eifect of the stray capacity@ on the tuned circuit 2 and 3 the series capacity 5 is provided. The capacity 5, however, causes a loss of coupling and in order to compensate for this loss the shunt inductance t is provided which together with stray capacity i and the capacity of a preset condenser which may be employed if the value of the stray capacity is not of the required value, forms a parallel resonant circuit tuned to about 550 kilocycles per second so as to improve the aerial performance at the low irequency end of the medium waveband.

In the long waveband position of the switch as shown in Fig. 2, variable capacity 3 is shunted across inductance 6 which now function as the tunable input circuit for the receiver, inductance 2 being short-circuited. In this condition the aerial l is connected directly to the end of the inductance t, the capacity Il having little undesirable effect owing tothe smaller band width of the long wave tuning range.

Although the invention has been described above as applied to an aerial coupling circuit, it will be understood that, if desired, the coupling circuit according to the invention, can be used as an interstage coupling.

What l claim is:

l. In a multi-band receiver, irst and second coils, a variable condenser, 4switch means for selectively connecting said condenser across each of said coils for tuning the receiver through different wavebands, a signal source, means for coupling said source to the rst of said coils, and a capacity Connected between the high potential sides of -said coils, the arrangement being such that in'one position of the band-switch means the variable condenser and the second coil constitute the tunable circuit of the receiver for one waveband and the rstcoil forms a parallel resonant circuit with a capacity which is constituted in whole or in partby stray capacity, said parallel resonant circuit being capacity coupled to the tunable circuit, and inv another position of the band-switch means 'the variable condenser and the first coil constitute the tunable circuit of the receiver for another waveband and the second coil is short-circuited.

2. In a multi-band receiver in accordance with the invention dened in claim 1 wherein the lrst and second switch positions correspond respectively to medium and long waveband `positions, and in the mediurn waveband position the parallel resonant circuit is tuned to a frequency at the low frequency end of said band.

4. 1n a multifband receiver, first and second coils, a variable condenser, switch means for selectively connecting said condenser across each of said coils for tuning the receiver through different wavebands, a signal source having stray capacity, means for coupling said source to the iirst of said coils, and a capacity `connected between the high potential sides of said coils, the arrangement being such that in one position of the bandswitch means the variable condenser and the second coil constitute the tunable -cir-cuit of the. receiver for a medium waveband and theV rst coil forms a parallel resonant circuit with a capacity which is constituted in whole or in part by the stray capacity of 'the signal source, said parallel resonant -circuit being tuned to a frequency at the low end of said medium waveband and capacity coupled to theA tunable circuit, and in another position of the band-switch' means the variable condenser and the first coil constitute the tunable circuit of the receiver for a long waveband, directly coupled to the signal source,

and the second coil is short-circuited.

4f. In a multi-band receiver verticalrod antenna as the signal source, iirst and second coils, a variable condenser, switch means for selectively connecting said condenser across each of said coils for tuning the receiver through diierent frequency bands, direct coupling from the rod antenna to the rst of said coils, and a capacityconnected between the high potential sides of said coils, the arrangement being such that in one position of the band-switch means the variable condenser and the second coil constitute the tunable circuit of the receiver for one frequency band and the rst coil forms a parallel resonant circuit with a capacity which is constituted in Whole or in part by the stray capacity of the rod antenna; said ,parallel resonant circuit being tuned to a irequency at the low end of said one frequency band and 4capacity coupled to the tunable circuit, and in another position of the band-switch means the variable condenser and the rst coil constitute the tunable circuit of the receiver for a lower frequency band and the second coil is shortcircuited.

CHARLES REFERENCES errno The following references are of record in the le of this patent:

UNITED STATES PATENTS WILLIAM EGGLETON,

Number Name Date 978,606 Marriott Dec. 13, 1910 1,805,942 Ferri-s May 19, 1931 1,813,724 Beste July 7, 1931 1,893,675 Mahone v J an. 10,v 1933 2,115,655 Thompson Apr. 26, 1938l 2,145,676 Zepler Jan. 31, 1939 FOREIGN PATENTS Number Country Date 284,149 Great Britain .v Jan. 26, 1928 utilizing a shortk 

